Roger Waters has reportedly begun work on a new rock opera, his first original album in more than two decades. With a £285m world tour behind him, the former Pink Floyd musician has completed initial recordings for what he describes as a kind of "radio play".

"I finished a demo of it last night," Waters said in a new interview with Rolling Stone. "It's 55 minutes long. It's songs and theatre as well. I don't want to give too much away, but it's couched as a radio play. It has characters who speak to each other, and it's a quest. It's about an old man and a young child trying to figure out why they are killing the children."

Waters has been tied up with a world-spanning revival tour of The Wall for the last three years. It ended in September. "I can't top that tour," he said. But he has been inspired by the satisfaction and success of that 219-date jaunt around the globe: "You have to accept the fact that I'm not going to live forever. I'm 70 years old … The hardest thing in the world is thinking of something to do, so going and doing it is a reward in itself."

Waters' last album was Amused to Death, released in 1992, which reached No 8 on the UK albums chart. Since then, his only new full-length was a 2005 soundtrack record for Ça Ira, an opera written with Étienne Roda-Gil and Nadine Delahaye. Speaking to the BBC in September, Waters said he wanted to make "at least one [more] record"; this new LP, he maintains, comes from a "very, very strong idea".

Still, Waters hasn't completely written off the idea of resuming The Wall tour at a future date. "I think there's an audience there," he said. "[But] I'm not thinking about that right now."